1. Introduction

Radio-frequency signals generated from optical sources have attracted considerable interest in both civilian and military applications, such as high-speed communications, sensors and radar systems [1–5]. Compared to other optical approaches including injection locking [6], optical phase-lock loop [7], and dual wavelength lasers [8], beating two optical signals which have consistent wavelength spacing is considered to be an attractive solution due to broad bandwidth and low phase noise [9-10]. Generally, there are two methods to achieve beat frequency output. One method is to spectrally combine two single-frequency laser signals from two different laser sources. An extra phase-locking technique is required and the stability of the frequency space is hard to be guaranteed [11]. Alternatively, radio-frequency signals can be created though beating two beams from the same single-wavelength laser. This method is using an acoustic-optical modulator (AOM) to shift the frequency of one of two beams which are split from the same single-frequency laser source and then these two beams are recombined to exploit beat frequency output. These two beams are generated from the identical seed laser, therefore the frequency of radio-frequency signal is well stable and the frequency can be conveniently tuned in a range of several ten MHz by tuning the drive signal of AOM [12–14].

In order to apply the radio-frequency signals into practice, high power, high signal-to-noise ratio (SNR) have to be developed. The output power of dual-frequency modulated signal though AOM is limited to tens of miliwatts up to now, which restrict the applications in long-distance transmission region. Due to low quantum defect and available high power pump laser diode (LD) source, the master oscillator power amplifier (MOPA) configuration based on Yb-doped fiber has been proposed to boost the output power of the laser signal [15–17]. Meanwhile, because of almost identical emission and absorption cross section in such narrow frequency gap, the two frequencies would be equally amplified maintaining the characteristics in the frequency domain. In 2014, Tao He fabricated a 50 mW tunable radio-frequency signal generation scheme and amplified it to 10 W utilizing a diode laser (LD) pumped 20/400 large mode area (LMA) fiber amplifier [12]. In 2016, his group promoted it to 50 W via a larger mode area fiber (25/250) [13]. Huang Long used a multimode fiber with core/inner cladding diameter of 30/250 to achieve 434 W dual-frequency laser signal output [14]. It should be noted that the phenomenon of mode instability is appeared when the power exceeds 279 W due to using such large mode area fiber.

Previous works mostly focus on adopting multimode fibers as the gain medium for the power scaling. Although the power over 400 W is obtained [14], the appearance of mode instability would reduce the brightness and beam quality of the laser. In practical application like lidar-radar system, except the output power, the brightness and beam quality of the laser source should also be considered. They would affect the energy of the radar echo signal, thereby impacting the detectable range and detecting precision [18–20]. Single-mode fiber could guarantee single spatial mode operation, but the output brightness is limited by the nonlinear effect, especially Stimulated Brillouin scattering (SBS) effect [21]. Suppressing the SBS effect is a critical technique to scale the brightness of the single-mode fiber laser for lidar-radar application.

In this paper, using a single-mode polarization maintaining (PM) fiber, a high brightness and high SNR linear-polarization optically carried radio-frequency signal based on all-fibered MOPA configuration is demonstrated. For the suppression of SBS effect in such small core diameter, a designed step-distribution strain is applied on the active fiber. The seed signal is obtained by combining two different frequency beams which are split from the same single frequency linear-polarization laser. One beam has initial frequency and the other beam is shifted by 200 MHz using an AOM. The radio-frequency signal of 200 MHz is generated due to the beat frequency of these two beams when they are recombined into a common chain. Then the dual-frequency signal is amplified by a LD pumped PM Yb³⁺ doped silica fiber. As a result, up to 143 W output power is achieved with the slop efficiency of 81.4%. The beam quality factors (M²) are measured to 1.06 ($(M_x^2)$) and 1.04 ($(M_y^2)$) and the SNR is up to 54.7 dB. The brightness of this dual-frequency laser is 103$W/\mu m^2/str$, which is twice as high as that in previous reports [14]. Moreover, thanks to this all-fibered PM single-mode structure, the SNR, polarization degree and modulation depth are well maintained after the amplification. To the best of our knowledge, this is the highest reported brightness of the optical-generated radio-frequency signal. These techniques would be also extended to the larger mode area fiber amplifiers, thus further scaling output brightness.

2. Experimental setup

The experimental setup based on the fiber MOPA structure is shown in Fig. 1.

 

Fig. 1 Experimental setup of monolithic fiber amplifier system (DBR, distribution Bragg reflection; AOM, acoustic-optical modulator; ISO, isolator; PA, preamplifier; LD, laser diode; YDF, Yb-doped double-cladding fiber; CLS, cladding light stripper; CO, collimator).

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A DBR laser operating at 1064 nm with an output power of 30 mW and a linewidth of 2 MHz is used to provide the single-frequency linear-polarization seed. Then it is split into two beams by a 2:8 single-mode splitter. One beam from the 20% output port is transmitted through a single mode fiber with initial frequency of ${\nu }_1$and the other beam from the 80% output port is injected into an AOM. The AOM is driven by a radio frequency (RF) signal with the frequency of ${\nu }_a=200\text{MHz}$and the frequency of its first-order diffraction light is${\nu }_2={\nu }_1+{\nu }_a$. Then these two ports are coupled into a$\text{2}\times \text{1}$fiber combiner and a radio-frequency signal with${\nu }_a$frequency is generated though beating these two beams. The beat signal can be expressed as

In order to boost the output power of the dual-frequency laser signal, an all-fibered MOPA structure is built up. The dual-frequency seed laser is amplified by 2-stage 10/125μm PM Ytterbium-doped double-cladding fiber (YDF) pre-amplifier chain to 2 W and then injected into the main amplifier. Between the preamplifier and the main amplifier, a high power PM optical isolator is inserted to block off the backward light from the main amplifier and a coupler with 99% and 1% coupling ratios is used to monitor the power and spectrum of the backscattered light. The main amplifier is based on a 3.7 m PM YDF with a core/inner cladding diameter of 10/125μm and pumped by six LD with 976 nm central wavelength. The cladding absorption coefficient of the active fiber is about 4.7 dB/m at 976 nm and the numerical aperture (NA) of the core/cladding is 0.075/0.46. After the active fiber, a PM cladding light stripper (CLS) and a high power fibered collimator (CO) with the total fiber length of 50 cm are fused for stripping the residual pump/cladding light and collimating the output light, respectively. A beam splitter is used to split 0.1% of the collimated laser to an optical spectrum analyzer (OSA) or a Fabry-Perot interferometer (FPI) to detect the output spectrum and the frequency characteristic.

3. Results and discussion

3.1 dual-frequency laser seed

The power of the dual-frequency seed laser signal from the output port of the 5:5 coupler is 10 mW, which is lower than input power due to the insertion loss of AOM and fiber splice. Output power ratio of the two frequency components is set as 1:1 to obtain a maximal modulation depth. Before amplification, the characteristics of dual-frequency seed laser in time domain and frequency domain are measured. Figure 2 (a) shows the scanning spectrum of the dual-frequency seed laser. It is measured by a FPI with 4 GHz free spectral range (FSR) and 10 MHz fineness. The blue line is a trigger signal of the saw-tooth wave which is used to repetitively scan the length of the cavity in order to sweep through at least one FSR of the interferometer. Two separate peaks with a frequency gap of 200 MHz which is displayed by the black line are correspond to the frequencies of the initial and the shifted beam. The almost identical intensity of the two beams verifies that the beat frequency signal has a high modulation depth. The beat signal is also monitored by a photo diode and oscilloscope, as shown in Fig. 2(b). It was shown that the frequency of the beat signal remains 200 MHz and the waveform of the signal is stable and smooth. Figure 2(c) depicts the Fourier frequency spectrum of the beat signal using a spectrum analyzer from 125 MHz to 275 MHz. It is noted that the SNR is about 52.4 dB, demonstrating a high stability of the radio-frequency signal.

 

Fig. 2 (a) Scanning spectrum of the dual-frequency seed laser; (b) oscillogram of the radio-frequency signal; (c) Fourier frequency spectrum.

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3.2 Suppression of the SBS effect in the main amplifier

Due to the relative small core of the single-mode active fiber, power scaling of the narrow linewidth MOPA is primarily limited by the onset of SBS. The SBS process is a third-order nonlinear effect that couples acoustic phonons in the fiber medium to the optical field and its associated backscattered Stokes field [22]. The effective peak SBS gain coefficient has a typical value of $3\times {10}^{-11}m/W$in silica-based fibers, making it the lowest threshold nonlinear process in narrow linewidth fiber amplifiers. However, if we externally applied thermal or stress gradients on the fiber surface, the Stokes bandwidth would be effectively broadened and the SBS threshold is equivalently increased. Therefore, for further suppressing the SBS effect in main amplifier, a longitudinally step-distributed strain with 20 steps is applied onto the gain fiber in our experiment. Followed with the analysis in [23], the active fiber is divided into 21 segments with the start and end parts unstrained. The maximum strain of 1.9 kg is applied to the second part and decrease the strain tension of 0.1 kg gradually until fiber end. The SBS gain spectra of each fiber segment are shifted respectively and have no overlap to each other. As a result, the effective SBS gain spectra are broadened and the efficient peak SBS gain is reduced. Considering the co-pump-induced temperature distribution, the maximal strain is applied on the input part of the gain fiber and is gradually decreased along the fiber. In addition, the input and output segments are unstrained for fusion with the combiner and CLS. The calculated strain distribution along the fiber and the broaden SBS spectra are depicted in Fig. 3. It is shown that the effective SBS gain spectrum is broadened from 40 MHz to 1.55 GHz with an improvement of about 20 times. Correspondingly, the effective peak SBS gain coefficient is reduced and the SBS threshold can be anticipated to be increased.

 

Fig. 3 (a) Strain distribution along the active fiber. (b) SBS gain spectra of strained and unstrained fiber.

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3.3 High brightness amplification of the radio-frequency signal

Then the dual-frequency seed laser is amplified by the 3-stage all-fibered linear-polarization amplifier, where the power is amplified to 150 mW by the first stage, 2 W by the second stage and the last stage boosts the dual-frequency to a higher power. Figure 4(a) shows the increase trends of the output power and backward power under varied pump power. The maximum output power of 143 W with 81.4% slop efficiency is attained at the pump power of 175 W. Figure 4(b) depicts the spectral content of the backward light. Note that at the maximal output power, the Stoke peak is still 5 dB lower than the Rayleigh peak. This infers that the SBS threshold has not been reached and the main amplifier still has the potential for further power scaling if additional pump source is available. It is also indicated from the backward power of Fig. 4(a), where the backward power is only 12 mW and still linear with respect to the forward power. The spectral content of forward light at the maximum output power is also measured by the OSA with a resolution of 0.02 nm, as shown in Fig. 5(c). The intensity of ASE is about 53 dB lower than that of the laser signal and no parasitic light is observed.

 

Fig. 4 (a) Output power and backscattering power with the pump power. (b) Backward light spectral content. (c) Forward light spectral content. (d) M² measured result and beam profile.

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Fig. 5 (a) Scanning spectrum of the dual-frequency seed laser; (b) Oscillogram of the radio-frequency signal; (c) Fourier frequency spectrum.

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After the main amplification, the characteristics of dual-frequency laser are measured. Figure 5(a) is the scanning spectrum of the dual-frequency laser at the maximal output power by FPI. There are two separate peaks with a frequency gap of 200 MHz and the intensities of the two beams are almost identical. It indicates that there is no gain competition between the two frequencies and both signals are amplified independently. Figure 5(b) shows the beat signal from the optical diode on the oscilloscope. It can be seen that the frequency separation and stable profile retain the same before and after amplification. The Fourier frequency spectrum of the beat signal is shown in Fig. 5. The SNR of the signal is ~54.7 dB which is almost same with the seed laser which proves the beat signal is stably maintained during the amplification.

The details of the Fourier spectra of the dual-frequency laser before and after amplification are shown in Fig. 6. Note that there are no symmetrical peaks beside the central laser frequency which are appeared in [13] at 50 dB degree. This is attributed to the relatively low phase noise of the seed laser and high maintaining SNR due to adopting the reliable all-fibered PM single-mode fiber amplifier chain. Existence of the symmetrical peaks would affect the SNR of the radio-frequency signal, thereby have impact on the detectable range and detecting precision. Therefore, this purer dual-frequency laser with stable state and low noise has more advantage as optically generated radio-frequency signal in practical applications.

 

Fig. 6 The Fourier frequency spectra details of the dual-frequency laser (a) before and (b) after amplification.

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As mentioned before, in practical application like lidar-radar system, except the output power, the brightness and beam quality of the laser source affect the detectable range and detecting precision. Therefore, we measured the beam quality of the dual-frequency laser by the M² beam analyzer. The measured results are shown in Fig. 4(d). Thanks for the single-mode active fiber in main amplifier, even the amplifier operates at full power level, the$M_x^2$and$M_y^2$ are 1.06 and 1.04. Results have shown that a single spatial mode output is obtained. The polarization ratio (PER) is also measured by a$\lambda /2$wave plate and a polarization beam splitter as 18 dB, indicating a high linear polarization output.

4. Conclusion

In summary, a high brightness high SNR linear-polarization optically carried radio-frequency signal based on all-fibered single-mode MOPA configuration is built and demonstrated. A 10 mW dual-frequency laser signal with beat frequency of 200 MHz is obtained by combining unshifted-frequency beam and shifted-frequency beam by AOM. Then the dual-frequency signal is amplified by a three-stage linear-polarization fiber amplification. More than 143W dual-frequency laser power is achieved, corresponding to a slop efficiency of 81%. The beam quality factors (M²) are measured to 1.06 ($M_x^2$) and 1.04 ($M_y^2$) and the SNR is up to 54.7 dB. The two frequency components with certain frequency gap can be identically amplified via the fiber amplifier and the beat note stability, modulation depth as well as SNR are maintained well before and after amplification. Due to adapting single mode fiber in the fiber chain, single transverse mode output makes the brightness of this dual-frequency laser is twice as high as that of previous reports. Low noise, high brightness makes this optically generated radio-frequency signal available for the applications in long distance raging, lidar-radar and imaging systems.